(a) Field of the Invention
The present invention relates to a quadrature VCO (voltage controlled oscillator.) More specifically, the present invention relates to a low-power quadrature VCO using a back gate of a switching transistor.
(b) Description of the Related Art
A quadrature VCO (voltage controlled oscillator) is a circuit for generating four kinds of signals with the same magnitude but with delays of 90 degrees respectively, and is generally applied to direct conversion transmitters and receivers.
FIG. 1 shows a block diagram of a quadrature VCO.
As shown, the quadrature VCO comprises first and second coupled delay cells 10 and 20.
In detail, signals output by (−) and (+) output terminals of the first delay cell 10 are applied to (+) and (−) input terminals of the second delay cell 20, and signals output by (−) and (+) output terminals of the second delay cell 20 are applied to (−) and (+) input terminals of the first delay cell 10.
According to the above-noted configuration, the (−) and (+) output terminals of the first delay cell 10 output signals with the same magnitude and phases of 90° and 270°, and the (+) and (−) output terminals of the second delay cell 20 output signals with the same magnitude and phases of 0° and 180°.
FIG. 2 shows a detailed diagram of a conventional circuit used as the first and second delay cells 10 and 20 in the quadrature VCO of FIG. 1.
As shown in FIG. 2, the first and second delay cells 10 and 20 comprise differential VCOs (voltage controlled oscillators) 11 and 21 for varying frequencies of output signals according to control voltages Vctr1 and Vctr2, and first to fourth coupling transistors MN25 to MN28 for coupling the first and second delay cells 10 and 20, the configuration and operation of which will now be described.
The differential VCO 11 of the first delay cell 10 comprises first and second switching transistors MN21 and MN22, first and second inductors L21 and L22, and first and second varactors Cv21 and Cv22, and the differential VCO 21 of the second delay cell 20 comprises third and fourth switching transistors MN23 and MN24, third and fourth inductors L23 and L24, and third and fourth varactors Cv23 and Cv24.
The first to fourth switching transistors MN21 to MN24 generate negative resistance of the differential VCOs 11 and 21, and are cross-coupled.
The first to fourth inductors L21 to L24 and the first to fourth varactors Cv21 to Cv24 form an LC tank, and vary impedance of the LC tank according to the applied control voltages Vctr1 and Vctr2, thereby varying the frequency of output signals.
As shown in FIG. 2, the conventional VCO additionally uses first to fourth coupling transistors MN25 to MN28 in order to couple the first delay cell 10 to the second delay cell 20. The coupling transistors MN25 to MN28 consume additional power which reaches 30 to 100% of power consumption of the switching transistors MN21 to MN24.
Also, since the active elements such as the MOS transistors cause noise, usage of first to fourth coupling transistors MN25 to MN28 deteriorates phase noise characteristics of the whole circuit.